The mineralogy, mineral chemistry, and origin of the Wuyang banded iron formations, North China Craton

摘要

Several banded iron formations (BIFs) occur in Precambrian strata of the North China Craton (NCC). Two subtypes of BIF occur in the Wuyang area, quartz-magnetite (QMB) and pyroxene-magnetite BIFs (PMB). However, the genesis of pyroxene and the how the BIFs formed are not well understood. Magnetite grains in QMB are relatively large with a straight boundary and sharp corners, and display a pronounced alignment, whereas magnetite in PMB is disseminated and has a smaller grain size. X-ray diffraction (XRD) analyses indicate that the crystal parameters of magnetite in QMB are: a = b = c = 8.396 angstrom and z = 8, whereas those of the magnetite in PMB are: a = b = c = 8.394 angstrom and z = 8. These parameters are consistent with magnetite forming in sedimentary iron formation. Based on scanning electron microscopy and energy dispersive spectroscopy (EDS) results, magnetite of both BIFs possesses high stabilities of geochemical property. Compared with magnetite in PMB, magnetite in QMB has higher concentrations of SiO2, Na2O, Al2O3, CaO, K2O, MgO, Sc, Co, Y, Nb, Ta and U. The enrichment of these elements is consistence with their derivation from seawater due to the strong adsorption capability of precursor iron hydroxide. A spider diagram of trace elements reveals that magnetite in QMB is enriched in Ta, whereas magnetite of PMB is depleted in Ce. Based on Raman and electron probe microanalysis of pyroxene in PMB, synchronous and compositional zoning are found in the degrees of crystallinity (FWHM values of Gaussian fitting for Raman shift next to 1014 cm(-1)) and the concentration of SiO2, Al2O3, MgO, MnO, CaO and FeO of pyroxene from core to rim. The two types of BIF formed in different sedimentary environments from different precursors. The precursor to the QMB BIF is almost pure chemical sediment, whereas a that for PMB contains a volcanic ash component. The QMB was likely deposited during a period of hydrothermal sedimentation in the Wuyang basin that was associate